Indeed it is, you’re right. Here is the spectrum the WHO publishes: https://www.ncbi.nlm.nih.gov/books/NBK574582/table/fm-ch1.tab1/?report=objectonly https://ww

Indeed it is, you’re right. Here is the spectrum the WHO publishes: https://www.ncbi.nlm.nih.gov/books/NBK574582/table/fm-ch1.tab1/?report=objectonly https://www.ncbi.nlm.nih.gov/books/NBK574582/table/fm-ch1.ta... Note that the idea of a single safe “threshold” limit is based on the best available research today showing that they haven’t measured adverse health effects below 5 μg/m³ average annual exposure. The spectrum here is a spread of increased mortality that can be attributed to pollution above that limit. There’s a different threshold for a single day exposure than for average exposure. Also note that the recommendation recently came down from the 2005 recommendation of 10 μg/m³ due to more research showing adverse health effects of pollution. There might not exist any so-called “safe” limit, and speculating wildly, we might expect their recommendation to drop again in the future.